GetToken uses filler to interact with the user and uses the provided
information to obtain an OAuth token from Ubuntu SSO. The returned
token can subsequently be used with LoginWithToken to perform a login.
The tokenName argument is used as the name of the generated token in
Ubuntu SSO. If Ubuntu SSO returned an error when trying to retrieve
the token the error will have a cause of type *usso.Error.

A StoreTokenGetter is a TokenGetter that will try to retrieve the
token from some storage, before falling back to another TokenGetter.
If the fallback TokenGetter sucessfully retrieves a token then that
token will be put in the store.